This invention relates to a thermoplastic resin composition. Specifically, it relates to lightweight thermoplastic resin compositions that possess superior rigidity, heat resistance and other desirable characteristics. The thermoplastic resin composition is useful as molding material to form molded articles such as those used as interior automobile parts.
In recent years, the automobile industry has been examining ways to reduce vehicle weight to improve fuel efficiency, which in turn reduces the carbon dioxide emissions that negatively impact the global environment. To reduce vehicle weight, the automobile industry has focused on various automobile parts, including interior parts, which are often molded from compositions containing thermoplastic resins. These thermoplastic resin compositions typically contain fillers, such as talc, mica, or various fibers, which also tend to be the heaviest components of the resin. Although these fillers increase the overall weight of the resin composition, they also provide the molded part with the desired rigidity and heat resistance properties. Consequently, simply reducing the quantity of filler to reduce weight often results in low, unsatisfactory rigidity and heat resistance standards in the molded part. Accordingly, achieving weight reduction by reducing filler quantity has generally been considered unacceptable.
Various thermoplastic resins have been suggested, such as those found in U.S. Pat. Nos. 5,998,524; 5,985,971; 5,777,020; and 5,773,515. However, all those resins contain octene. Additionally, U.S. Pat. No. 4,734,459 discloses a polypropylene composition containing polypropylene with an isotacticity index of  greater than 90 and an ethylene/butene-1 polymeric mixture. However, this composition exhibits poor heat resistance.
To reduce weight, Japanese patent application 5-279526 discloses resin compositions consisting of specific crystalline propylene/ethylene block copolymer, fibrous inorganic filler, talc, olefin elastomer and/or styrene elastomer. According to Japanese Patent Application 5-279521, such a resin composition provides a lightweight, high-quality resin composition that maintains sufficient rigidity, heat resistance and other physical properties, while achieving a significantly lower specific gravity and a good external appearance.
However, resin compositions such as in Japanese Patent Application 5-279526 require expensive filler materials, adding to already-expensive manufacturing costs. Thus, there still remains a need in the art to create an economical, lightweight, high-quality thermoplastic resin composition that maintains necessary physical properties such as rigidity and heat resistance. This invention answers that need.
In one embodiment, the invention relates to a thermoplastic resin composition. The composition contains (a) a crystalline propylene ethylene block copolymer or a combination of crystalline propylene ethylene copolymer and a propylene homopolymer whose (i) ethylene content ranges from about 2.2 to about 4.2 weight %, (ii) melt flow rate at 230xc2x0 C. under a 2.16-kg load ranges from about 20 to about 30 g/10 minutes, and (iii) isotactic/pentad fraction of propylene homopolymer, as measured by 13C-NMR, is greater than or equal to about 94%. The composition also contains (b) ethylene butene rubber whose melt flow rate ranges from about 6 to about 8 g/10 minutes and density ranges from about 0.860 to about 0.865 g/cc and (c) talc with an average diameter ranging from about 1 to about 2 xcexcm. Constituent (a) is present in the composition from about 85 to about 95 weight %. Constituent (b) ranges from about 2 to about 8 weight %. Constituent (c) ranges from about 2 to about 8 parts by weight.
In a second embodiment, the invention relates to a molded thermoplastic resin composition of the thermoplastic resin composition described above. In a third embodiment, the invention relates to a process of preparing a molded thermoplastic resin, comprising the steps of filling a molder with a thermoplastic resin composition as described above, and curing the molded thermoplastic resin composition to form a molded article.